OP, be graceful in your disappointment. Hopefully your sister and niece can still come?
I believe that weddings are important events that you cancel things for, particularly if you are the sister or nephew of the bride. I would cancel my children's concerts on the Strathmore stage or gym competitions in a heartbeat.
However, my kids are not into soccer tournaments, and I do not know if missing this tournament will crush your nephew's chances. If that is really the case, then we as a society have a problem!!! Competitive sports should not put such pressure on minors such that if they miss one game, they're done.
So now you know - don't enroll your kids in soccer